Seismic waves generated artificially have been used for more than 50 years to perform imaging of geological layers. During seismic exploration operations, vibrator equipment (also known as a “source”) generates a seismic signal that propagates in the form of a wave that is reflected at interfaces of geological layers. These reflected waves are received by seismic sensors, such as hydrophones, geophones or accelerometers, which convert the displacement or overpressure of the ground resulting from the propagation of the waves into an analog or digital, electrical or optical signal which is recorded. Analysis of the arrival times and amplitudes of these waves make it possible to construct a representation of the geological layers on which the waves are reflected.
FIG. 1 depicts schematically a system 100 for transmitting and receiving seismic waves intended for seismic exploration in a land environment. The system 100 comprises a source 102 consisting of a vibrator operable to generate a seismic signal, a set of receivers 104 (e.g., geophones) for receiving a seismic signal and converting it into an electrical signal and a seismic data acquisition recorder system (recorder system) 106 for recording the electrical signals generated by the receivers 104. The source 102, the receivers 104 and the recorder system 106 are positioned on the surface of the ground 108. FIG. 1 depicts source 102 as a single vibrator but it should be understood that the source can be composed of several vibrators, as known to those of skill in the art. System 100 further includes vehicle 122a, for housing the source 102, and vehicle 122b for housing recorder system 106, as well as antennas 124 for communications between vehicles 122a,b (and source 102) and receivers 104. The receivers 104 are interconnected by cables 126 and connected to recorder system 106. Antennas 124 on receivers 104 can communicate data from receivers 104 to recorder system 106, as can cables 126. Furthermore, in operation, vehicle 122a is generally not static, but generates transmitted waves in different locations for the geographical area of interest (GAI).
In operation, source 102 is operated so as to generate a seismic signal. This signal propagates firstly on the surface of the ground, in the form of surface waves 110, and secondly in the subsoil, in the form of transmitted body waves 112 that generate reflected and converted waves 114 when they reach an interface 115 between two geological layers. Each receiver 104 receives both a surface wave 110 and a reflected wave 114 and converts them into an electrical or optical signal, which signal thus includes a component associated with the reflected wave 114 and another component associated with the surface wave 110. Since system 100 intends to image the subsurface regions 116 and 118, including a hydrocarbon deposit 120, the component in the electrical signal associated with the surface wave 110 is usually undesirable and should be filtered out.
An example of a vibratory source (source) 102 is shown in FIG. 2. Source 102 can include base plate 288 and reaction mass 284, and actuator 250 that applies a force between base plate 288 and reaction mass 284. Actuator 250 can be hydraulic and can consist of rod 280 and piston 282 inside reaction mass 284, in which case the force is generated by injecting into one of the piston chamber a pressurized fluid, usually oil, while the same fluid is drawn from the other chamber. A four way valve can adjust the circulation of the pressurized fluid, and a control system (not shown) can set the position of the valve so that a desired force is applied to the ground by baseplate 288. A hydraulic pump driven by a motor (also not shown) can supply the pressurized fluid necessary for the operation of source 102, while hydraulic accumulators can be used to supply limited amounts of oil and limit the variations of flow rate through the pump. In another implementation, actuator 250 can be a plurality of electric linear motors. The electric current sent to the individual electric linear motors can be set by a control system, so that the force applied to the ground by baseplate 288 reaches a desirable level. In another implementation, the actuator 250 can be a piezoelectric actuator. The electric voltage applied to piezoelectric actuator 250 can be set by a control system, so that the force applied to ground 275 by baseplate 288 reaches a desirable level. Vibration isolation devices 286 can be provided on base plate 288 to transmit weight 290 of vehicle 122a to base plate 288. Base plate 288 is shown in FIG. 2 as contacting the ground 275. The force transmitted to ground 275 can be estimated by a “weighted sum estimate” method as equal to the mass of base plate 288 times its acceleration, plus the weight of reaction mass 284 times its acceleration. Accelerometers located on baseplate 288 and reaction mass 284 provide to the control system a feedback on the force applied to ground 275 by baseplate 288. The weight of vehicle 273 (shown in FIG. 1 as vehicle 122a) prevents base plate 288 from losing contact with ground 275. Those skilled in the art will appreciate that many other designs for vibratory sources 271 exist on the market, and that any of them can be used with the embodiments discussed herein.
Vibratory source 271 generates a long tone with a varying frequency, i.e., a frequency sweep. This desired signal is input into the control system, which then sets the parameters of actuator 250 applied to a moving part, e.g., a piston, which generates a corresponding seismic wave. More specifically, the control system adjusts some parameters of the actuator 250 (depending on, for example, the type of actuator being used: hydraulic, electric, etc.) so that the force exerted by the baseplate onto the ground matches a desired signal. Applying a variable force onto the ground results in seismic waves being radiated at the surface of the ground and into the subsurface. The design of the frequency sweep for a seismic system will now be discussed.
A frequency sweep is a sinusoid with a continuously variable frequency, and can be defined by its amplitude A(f) and its sweep rate Sr(f), the latter of which is defined as the derivative of the frequency relative to time df/dt. Provided the sweep is long enough (e.g., longer than 5 or 6 seconds), the amplitude spectrum of the sweep at frequency f is proportional to A(f) and to the square root of 1/Sr(f). Target-oriented sweep design (i.e., searching for a particular known type of hydrocarbon, in a particular known type of geological formation) involves defining A(f) and Sr(f) to obtain the desired signal-to-noise ratio (SNR) of the target reflection.
It has thus become common to use seismic vibrators (sources 271) in seismic prospecting with predetermined frequency sweeps. Typically, a number of vibrators 271 emit a long swept frequency signal in the seismic frequency range. The emitted signal 112 (after reflection and refraction within the earth) is received by detectors 104 spaced along a spread, and received signal 114 can then be cross-correlated with the emitted swept frequency signal 112. This operation yields a seismic record that is then processed into a representation of a cross-section of the earth, using techniques known to those of skill in the art.
The main factors that limit the amplitude of signal 112 that vibratory source 271 can send into ground 275 have been identified as the peak force, which is the maximum force vibrator 271 is capable of applying between baseplate 288 and its reaction mass 284 while remaining coupled to ground 275, and the mass stroke, which is the maximum allowable displacement of reaction mass 284 with respect to baseplate 288. While “pushing” onto ground 275 is usually not an issue, “pulling” baseplate 288 too hard, in excess of the hold-down weight 290, will result in baseplate 288 lifting from ground 275.
The mass stroke, which is the maximum allowable displacement of reaction mass 284 with respect to baseplate 288, is another limitation. As the reaction mass 284 moves freely, any force applied between baseplate 288 and reaction 284 mass will result in a movement of the reaction mass 284. The space in which reaction mass 284 moves is limited, so there is a maximum displacement that reaction mass 284 should not exceed.
In the case of hydraulic vibrators, the oil flow may also be a limitation. The average oil flow needed to operate vibrator 102 at a certain frequency and amplitude cannot exceed the rated flow of the pump in steady-state. The peak oil flow through the four-way valve can also be a limiting factor, as a high flow rate of oil can generate a pressure drop that reduces the force applied to reaction mass 284, and as a high flow rate may result in accelerated erosion and wear of the valve. How these factors affect the signal depends on the frequency. Specifically, the peak force will typically be the limiting factor at high frequencies, the mass stroke at low frequencies and, with a hydraulic vibrator 102, the average oil flow rate for mid-range frequencies.
There exist already methods to maximize the amplitude of the emitted signal depending on which of these factors is the limiting factor, and these methods depend on the characteristics of vibrator 271 and on the emitted frequency, e.g., as described in U.S. Pat. No. 8,274,862, to John Sallas. However, such systems do not typically describe a mechanism to actually increase useful source output without exceeding system limits.
While it is the case that the emitted seismic signal will always be constrained to some extent by characteristics of the vibrator, it would nonetheless be desirable to emit a higher amplitude signal for given vibrator characteristics. Accordingly, it would be desirable to provide methods, modes and systems for increasing the amplitude of an emitted seismic signal without, for example, requiring increases in any of required force, oil flow rate and/or mass displacement with respect to seismic vibrators. Likewise, it would be desirable to provide methods, modes and systems which reduce the required force, oil flow rate and/or mass displacement with respect to seismic vibrators, while preserving the amplitude of the emitted signal.